: Type 1 diabetes is a T cell mediated autoimmune disease. Nonobese diabetic (NOD) mice develop diabetes that is similar to that in human. It is hypothesized that a breakdown in the thymus selection process and a biased development of antigen specific autoreactive Th1 cells in NOD mice, but not in disease resistant strains of mice, plays a critical role in autoimmune diabetes. NOD mice express a disease associated MHC Class II I-Ag7 that is necessary to be present in both alleles for diabetes development. An understanding of how various autoantigenic epitopes may select for autoreactive T cells, and detection and characterization of diabetogenic T cells should help us to determine the regulatory mechanisms leading to diabetes and to design therapeutic treatments. During the last application period, we have successfully generated autoantigen specific I-Ag7 tetramers to detect T cells. The major goals for this application are to continue our studies to detect and characterize T cells of different antigen specificities using I-Ag7 tetramers, and to determine their role in diabetes development. There are three specific aims: Experiments in Specific Aim A are designed to use the available I-Ag7 tetramers to isolate and characterize autoreactive T cells. The studies will allow us to determine if these cells can cause diabetes and to characterize their functions, such as their cytokine secretion pattern. In Specific Aim B, additional I-Ag7 tetramers specific for other autoantigenic peptides will be generated. These studies should help us to determine if autoreactive T cells with different antigen specificities may play varying roles in diabetes. In Specific Aim C, experiments will be performed to isolate and compare autoreactive T cells with the same antigen specificities from NOD mice with T cells from disease resistant strains. These experiments should allow us to understand the mechanisms of how autoreactive T cells arise in NOD vs. disease resistant mice. The overall proposed studies should help us to determine the roles of different autoantigenic peptides and autoreactive T cells, and regulatory mechanisms underlying immune responses leading to the disease.